This invention relates to an improvement in an atmospheric pressure ionization mass spectrometer, and particularly to an apparatus for removing clustor ions, suitable for efficient removal of clustor ions giving rise to sensitivity lowering and spectrum complication.
The atmospheric pressure ionization mass spectrometer is an apparatus very sensitive to gaseous substances and has now been practically utilized in the fields of pollution measurement, semiconductor production process and metabolite analysis. The atmospheric pressure ionization mass spectrometer is characterized by its high sensitivity, and thus it is important to eliminate factors inhibiting this high sensitivity.
A conventional atmospheric pressure ionization mass spectrometer is shown in FIG. 1, where a sample gas 15 is introduced into an ion source 3 through a sample inlet pipe 1, and a portion of the sample gas 15 is ionized under an ion source pressure of 1 atm. The thus formed ions are led to a low pressure region 9 through a differential pumping region 6. There is a quadrupole mass analyzer 7 in the low pressure region 9, and the ions are separated according to the masses, and reach an ion collector 8. The ion current obtained at the collector 8 is output to a recorder 12 and a computer 14 through an amplifier 13. The pressure in the low pressure region 9 is kept at about 10.sup.-4 Pa by the working pressure of the quadrupole mass analyzer 7. The differential pumping region 6 is provided to connect the low pressure region 9 to the ion source 3 under 1 atm, and is partitioned from the ion source 3 under 1 atm by a first aperture electrode 4 having an aperture through which the ions can pass and by a second aperture electrode 5 having an aperture through which the ions can pass.
Ionization of the atmospheric pressure ionization mass spectrometer is initiated by corona discharge at the tip end of a needle electrode 2 to which a high voltage is applied. Trace amounts of oxygen, carbon dioxide, and organic compounds (M) are contained in a nitrogen gas, through ionization as follows: ##STR1##
N.sub.2, which is a main component in the sample gas 15, is ionized according to the reaction (1), but the ions formed according to the equation (1) undergo the following reactions owing to the very short mean free path because the ionization is carried out under 1 atm. EQU N.sub.2 ++2N.sub.2 .fwdarw.N.sub.4 ++N.sub.2 ( 2) EQU N++2N.sub.2 .fwdarw.N.sub.3 ++N.sub.2 ( 3) EQU N.sub.4 ++O.sub.2 .fwdarw.O.sub.2 ++2N.sub.2 ( 4) EQU N.sub.4 ++CO.sub.2 .fwdarw.CO.sub.2 ++2N.sub.2 ( 5) EQU N.sub.4 ++H.sub.2 O.fwdarw.H.sub.2 O++2N.sub.2 ( 6)
Since the ionization potential of N.sub.4 is higher than those of O.sub.2, CO.sub.2, H.sub.2 O, etc., ions of trace components are formed in the nitrogen gas according to the reactions (4), (5) and (6). The main component ions which are not analytical objects are converted to trace component ions, which are analytical objects, as given by the reactions (4), (5) and (6), which also occur under 1 atm. Thus, there are many chances for the reactions, and a highly efficient ionization of the analytical trace components as the objects can be attained. Since these ions are detected in the analytical region 9 through the differential pumping region 6, the atmospheric pressure mass spectrometer can have a higher sensitivity. However, the following reactions occur to inhibit the higher sensitivity. EQU H.sub.2 O++2N.sub.2 .fwdarw.H.sub.2 O+.N.sub.2 +N.sub.2 ( 7) EQU H.sub.2 O+.N.sub.2 +2N.sub.2 .fwdarw.H.sub.2 O+.(N.sub.2).sub.2 +N.sub.2 ( 8) EQU H.sub.2 O+.N.sub.2 +H.sub.2 O.fwdarw.H.sub.3 O++OH+N.sub.2 ( 9) EQU H.sub.3 O++H.sub.2 O+N.sub.2 .fwdarw.H+(H.sub.2 O).sub.2 +N.sub.2 ( 10) EQU H+(H.sub.2 O).sub.n-1 +H.sub.2 O+N.sub.2 .fwdarw.H+(H.sub.2 O).sub.n +N.sub.2 ( 11) EQU H+(H.sub.2 O).sub.n- +M+N.sub.2 .fwdarw.M.H+(H.sub.2 O).sub.n +N.sub.2( 12)
The ions formed according to the reactions (8) to (12) are called cluster ions, which have the following disadvantages: the spectrum will be complicated, because, for example, the proper peak of water appears at m/z=18, whereas clustor ions develop peaks at other values of m/z, for example, H.sub.2 O+.N.sub.2 (m/z=46), H.sub.2 O+,(N.sub.2).sub.2 (m/z=74), etc., and S/N ratio will be lowered, because the properly single peak is divided into a plurality of peaks. Particularly, the lowering of S/N ratio will reduce the sensitivity, and thus the removal of the cluster ions is indispensable for an atmospheric pressure ionization mass spectrometer.
The prior art of removing the cluster ions, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 53-81289 proposes to provide a drift electric field in the differential pumping region 6 in FIG. 1 to make the clustor ions collide with neutral molecules, thereby dissociating the cluster bonds. That is, a voltage is applied between the electrode 4 and the electrode 5 to accelerate the cluster ions and make them collide with the neutral molecules. The kinetic energy of the cluster ion is converted to the internal energy by the collision, and if the number of collisions is enough, the cluster ions will be dissociated at the weak bonds. EQU H.sub.2 O+.(N.sub.2).sub.2 +N.sub.2 .fwdarw.H.sub.2 O+.N.sub.2 +2N.sub.2 ( 13) EQU H.sub.2 O+.N.sub.2 +N.sub.2 .fwdarw.H.sub.2 O++2N.sub.2 ( 14) EQU M.H+.(H.sub.2 O).sub.n +N.sub.2 .fwdarw.M.H+.(H.sub.2 O).sub.n-1 +H.sub.2 O+N.sub.2 ( 15) EQU M.H+(H.sub.2 O).sub.n +N.sub.2 .fwdarw.MH++H.sub.2 O+N.sub.2 ( 16)
The cluster bond energy is generally smaller than the chemical bond energy. Therefore the cluster bond is dissociated according to the reactions (13)-(16) and molecular ions are produced. In the prior art the pressure in the intermediate pumping region is constant (the number of collisions is constant), and thus the control of cluster bond dissociation has been so far carried out by controlling the kinetic energy, that is, by controlling the voltage applied to the electrodes 4 and 5 (drift voltage). However, when the drift voltage is increased to dissociate M.H+.(H.sub.2 O).sub.n clusters having a larger n in the prior art controlling method, the optimum conditions for focusing the ion beams into the aperture of the electrode 5 cannot be obtained, and thus the amount of ions to be introduced into the analytical region 9 is reduced. When the drift voltage is not to be increased, it is necessary to increase the number of collisions. When the pressure of the differential pumping region 6 is elevated to increase the number of collisions, the pressure of the low pressure region 9 will be increased and the aperture of the electrode 5 is liable to be fouled. This gives rise to charge-up and a consequential reduction in the amount of ions to be introduced into the low pressure region 9. These are serious disadvantages of the prior art.